FIG. 11 is a sectional view showing a conventional electrolytic capacitor. As shown in FIG. 11, lead wire 151 of a conventional electrolytic capacitor includes aluminum wire round rod 152 and external terminal 154 made of copper clad tin-plating iron wire (CP wire), which are joined to each other. Capacitor element 156 is produced by respectively connecting lead wires 151 to an anode foil and a cathode foil, which are made of valve action metal such as aluminum, and winding the anode foil and the cathode foil with a separator interposed therebetween.
Exterior case 160 is made of aluminum and has a cylindrical shape with a bottom. Exterior case 160 accommodates capacitor element 156 and an electrolyte (not shown) such as a driving electrolytic solution and a solid electrolyte of electroconductive polymer.
An open end of exterior case 160 is sealed with rubber elastic sealing member 161 having through-holes 162 through which a pair of lead wires 151 pass. Thus, an electrolytic capacitor is produced.
A method of manufacturing a conventional lead wire used for such an electrolytic capacitor includes joining aluminum wire round rod 152 and an end portion of external terminal 154 to each other by arc welding. Another method includes providing an end surface of aluminum wire round rod 152 with a hole and inserting external terminal 154 into the hole, followed by heating with a gas burner so as to join aluminum wire round rod 152 and external terminal 154 to each other.
Note here that Patent documents 1, 2 and 3 can be referred to for reference as a published document related to this application.
In such a conventional method of directly joining aluminum wire round rod 152 and CP wire of external terminal 154 by arc welding, when the diameter of aluminum wire round rod 152 is reduced and the difference between this diameter and a wire diameter of external terminal 154 is reduced, it becomes difficult to control the welding condition. Consequently, aluminum wire round rod 152 and external terminal 154 tend to be melted excessively, thus causing the formation of burrs and protrusions at the joint portion.
Such burrs and protrusions in a lead wire cause abrasion in a through-hole when a lead wire is inserted into the through-hole of a sealing member. This produces gap between the lead wire and the sealing member, resulting in leakage of a driving electrolytic solution. As a result, the reliability of an electrolytic capacitor is reduced.
Patent document 1: Japanese Patent Unexamined Publication No. H6-310385
Patent document 2: Japanese Patent Unexamined Publication No. H6-349687
Patent document 3: Japanese Patent Unexamined Publication No. 2002-19829